The invention is a submersible laser scattering instrument that measures particle total volume, particle total area and Sauter mean diameter. It allows a single calibration for all particle sizes 1.2-250 xcexcm.
Prior Sensors for Sediments: In most cases, suspended sediment xe2x80x98concentrationxe2x80x99 has been estimated via one parameterxe2x80x94optical transmission, optical backscatter, or acoustic scattering cross-section. A one-parameter sensor necessarily obtains a weighted sum of the concentrations of underlying size classes. For example, optical transmission or backscatter sensors estimate approximate (not exact) total particle area. In contrast, acoustic sensors, usually operating in the Rayleigh regime [i.e,. when the insonifying acoustic wavelength xcexa is of the same order or greater than the particle diameter, i.e. kaa less than 1 where ka=2xcfx80/xcexa] respond to the sum of the squares of particle volumes. This condition is satisfied for particles of 1 mm diameter or smaller at acoustic frequencies of 1 MHz or lower. Neither of these sensors simply sum the mass or volume concentrations to provide the needed measure of Cn or the total concentration Cn. For this reason, unless the particle size distribution is invariant in space and time, the calibration of these single-parameter sensors in laboratories before field usage, while a common practice, is of limited value. The most unfortunate consequence of the use of such calibrations is the lack of even the error bounds in the interpretation of data. Certainly, historical data with these unknown errors are in part responsible for the large variability in predictive capability of sediment transport models.
Optics affords a capability to observe a wide range of particle sizes. By measuring optical scattering over a wide dynamic range of angles [dynamic range is defined here as the ratio of maximum to minimum scattering angle], a measurement is obtained with information content on a correspondingly large dynamic range in particle sizes. The angular dynamic range is typically 100:1 or 200:1 so that size ranges from, say, 1-200 microns can be studied with a single instrument. This principle is called laser diffraction. The name derives from the approximation to the exact solution to Maxwell""s equations describing light scattering by spheres. The exact solution for homogeneous spheres of arbitrary size, due to [Mie, 1908], has the property that for large particles, i.e. when the real part m of the complex refractive index, and particle size ka (k being 2xcfx80/xcex,xcex is optical wavelength) are such that (mxe2x88x921)ka greater than  greater than 1, the scattering at small forward angles appears nearly identical to the diffraction through an equal diameter aperture (see [Bom and Wolf, 1975]). An even more significant observation is that under these conditions, the refractive index of particles becomes largely irrelevant. This implies that the particle composition, or for that matter, possibly particle internal structure and homogeneity, are of little to no consequence. As the particle composition does not determine its scattering characteristics, the method is fully general for particle sizing. It is for this reason, that this has become the most widely used particle sizing method, employed for measuring diverse types of particles, including cements, chocolates or microbes.
The first underwater instrument based on laser diffraction was developed by [Bale and Morris, 1987]. They adapted a commercial laboratory instrument manufactured by Malvern Instruments of UK for ocean use. They have presented results from estuarine particle sizing [Eisma, 1996]. Recently, a team of French scientists has employed a submersible instrument manufactured by CILAS ([Petrenko et al., 1997], [Gentien et al., 1995], [Lunven et al., ]). Multi-angle scattering was observed using a CCD line array photo-detector ([Agrawal and Pottsmith, 1994]). The use of CCD""s unnecessarily required long averaging times to remove the influence of laser speckle, and also required complex, fast electronics.
The invention is referred to as the LISST-25. Based on laser diffraction technology, the LISST-25 is designed to record the total suspended particle area and volume concentrations, and the Sauter mean diameter of suspended particles. The main advantage of the LISST-25 is that, unlike transmissometers, optical backscatter sensors, or single-frequency acoustic sensors, the LISST-25 has a constant calibration over the covered range of particle sizes. The LISST-25 does not give the particle size distribution.
The technique used by the LISST-25 sensor differs from other turbidity sensors. It measures the forward scattered light from a collimated laser beam using only two specially designed silicon detectors. These measurements are used to obtain the total volume and total area of particles directly, bypassing inversion of data as is done in other instruments to obtain the particle size distribution. From the ratio of total volume and area, the Sauter mean diameter is obtained.
The LISST-25 is a self-contained instrument that includes optics and electronics, datalogger, and a battery pack. The on-board data logger allows for simple programming of sampling schedules and can be connected to a personal computer via an RS-232port. The depth rating is 300 meters. The instrument covers a wide dynamic range of concentrations, from 0.1 to 1,000 mg/l (microliters/liter). The operational limit of the instrument is based on optical transmission of water, i.e., beam c range between 0.5 mxe2x88x921 to 25 mxe2x88x921.
In one aspect, the invention is an instrument based on scattering of laser light for measuring both total particle volume and total particle area at the same time for particles suspended in water. A beam of laser light is directed across a void where a sample of water containing particles is admitted. After passing through the water, the light which is forward scattered out of the direct beam falls on two detectors at the same time. The first detector has an active surface shape which is configured to produce an output signal proportional to total particle area at varying total particle areas. The second detector has an active surface shape which is configured to produce an output signal proportional total particle volume varying total particle volumes.
Each detector falls within in a radius surrounding the unscattered beam of light. At each annulus within this circle, whether the annulus is described as having a finite width or an infinitely small width, the active surface shape of each detector intercepts less than the entire annulus. The active surface shape of the detector for area increases with distance from the axis of the beam at an increasing rate. The active surface shape of the detector for volume decreases with distance from the axis of the beam at a decreasing rate.
In the preferred embodiment, the output from each detector is directly proportional to the quantity to be measured. This is accomplished by the carefully designed shape of each detector. These two measured outputs are then electronically combined to obtain mean diameter for the measured particles.
In the preferred embodiment, the two detectors are both fabricated on a single semiconductor substrate plate made of silicon. Each detector is formed of conventional photodiode material deposited on the silicon plate. Alternatively, the invention may be embodied in two separate detectors on separate substrates. Whether formed on a single substrate or two substrates, instead of depositing the photodiode material on the substrate in the desired active surface shape, the preferred active surface shape may be achieved by placing a mask on top of photodiode material with an active surface area larger than the desired active surface shape. The mask can simply cover the periphery beyond the desired shape or it can be comprised of a pattern of dots or checkerboard or other shapes of alternating opaque and transparent portions. The opaque and transparent portions are configured such that only the desired portion of each annulus at a distance from the axis of the beam of light passes light to the active surface of the photodetector.
In a second aspect, the invention is an instrument which includes only a single detector, either one of the detectors described above.